Orthopaedic aids such as shoe inserts or corsets, typically for spinal correction in treating wrong posture, usually have to be shaped to a very specific form which is individually accommodated to a specific wearer.
To enable this accommodation to be made in simple manner, such orthopaedic aids have for some time been increasingly made from polymethacrylate (PMMA), i.e. a thermoplastic polymer. The procedure normally comprises first mechanically preforming a semifinished product made from the material, typically a polyamethacrylate board, by customary mechanical shaping methods such as cutting, milling, drilling, abrading or polishing. The final shaping, i.e. the exact matching of the blank so obtained to the requirements of a specific patient, is finally performed by thermoforming, i.e. by a shaping operation in which the material is heated so that it becomes elastic.
Polymethylmethacrylate has, however, a number of shortcomings in the above described process, which make it desirable to find a substitute material. On the one hand, PMMA begins to soften at quite low temperatures, so that mechanical processing can only be carried out with difficulty, as the tools easily become tacky or the material becomes soft and stringy. On the other hand, rather high temperatures of c. 180.degree. C. are required for the final thermoforming, as only after shaping at such high temperatures is a sufficiently low, degree of elastic memory ensured. Aside from the time and effort involved, the danger of hydrocyanic acid emission is always present when heating plexiglass material to such high temperatures.